The present invention relates to a Time of Flight mass analyser, a mass spectrometer, a method of analysing ions and a method of mass spectrometry. The preferred embodiment relates to an orthogonal acceleration Time of Flight mass analyser.
It is known to apply a DC voltage pulse to a pusher electrode located in an orthogonal acceleration region of a Time of Flight mass analyser. The DC voltage pulse which is applied to the pusher electrode generates a DC electric field which causes ions to be orthogonally accelerated from the orthogonal acceleration region into the drift region of the Time of Flight mass analyser.
Wiley and McLaren (Time-of-Flight Mass Spectrometer with Improved Resolution, (Review of Scientific Instruments 26, 1150 (1955), W C Wiley, I H McLaren) set out the basic equations that describe two stage extraction Time of Flight mass spectrometers. The principles apply equally to continuous axial extraction Time of Flight mass analysers and orthogonal acceleration Time of Flight mass analysers and time lag focussing instruments.
In a Time of Flight mass analyser it is preferable to optimise the geometry and amplitude of the electric fields so that the deviation in arrival times as measured at the detector, for ions of the same mass to charge ratio, is only a small function of the initial ion spatial coordinates.
Consider ions in the pusher field of an orthogonal acceleration Time of Flight mass analyser having a coordinate x (with no initial velocity component). The time of flight can be expanded spatially as follows as a function of x:ToF(x)=A+B·x+C·x2+D·x3  (1)
For a system spatially focused to first order, the coefficient B is arranged to be zero. This is achieved by choosing the appropriate amplitude and lengths of the electric fields and field free regions. For a system to be second order focused, the coefficients B and C are arranged to be zero. For a system to third order focused, the coefficients B, C and D are arranged to be zero and so on.
Most commercial systems are first and second order spatially focused and the spatial time of flight aberration is usually very small so as not to affect the overall resolving power of the system. However, available geometries required to achieve second and higher order focusing can be limited and can cause compromises in other areas of performance.
Another significant aberration on Time of Flight systems is known as the “turn around time”. This is caused by the initial velocity spread of ions prior to extraction with the pusher field. Consider two ions with the same initial kinetic energy but with equal and opposite velocity components. When the pusher field is applied, the ion with the negative velocity has to be turned around before it can set off in the time of flight direction. This turn around time limits the resolving power of commercial Time of Flight mass analyser systems.
The value of the aberration is 2·u/a wherein u is the initial velocity component and a is the acceleration of the pusher field.
The ions will be separated by a turnaround time Δt which is smaller for steeper acceleration fields. This is often the major limiting aberration in Time of Flight instrument design and instrument designers go to great lengths to minimise this term.
The most common approach to minimising this aberration is to accelerate the ions as forcefully as possible i.e. the acceleration term a is made as large as possible by maximising the electric field i.e. the ratio Vp/Lp. This is normally achieved by making the pusher voltage Vp large and the acceleration stage length Lp short. However, this approach has a practical limit for a two stage geometry as the Wiley McLaren type spatial focussing solution leads to shorter physical instruments which will have very short flight times. Very short flight times would require ultra fast high bandwidth detection systems and hence are impracticable.
A known solution to this problem is to add a reflectron wherein the first position of spatial focus is re-imaged at the ion detector. This leads to longer practical flight time instruments which are capable of relatively high resolution.
In conventional reflectron Time of Flight instruments the reflectron may comprise either a single stage reflectron or a two stage reflectron whilst in both reflectron and non-reflectron Time of Flight instruments the extraction region usually comprises a two stage Wiley/McLaren source. Usually within these geometries the objective is to achieve perfect first or second order space focusing or to re-introduce a small first order term to further improve space focusing.
It is known that a small first order term may be arranged to compensate for linear pre-extraction velocity-position correlations obtained in various ion transfer configurations.
Despite known approaches to space focusing, the practical performance of known Time of Flight instruments is limited by space focusing characteristics. These limitations are most evident in the relationship between resolution and sensitivity.    US 2002/100870 (Whitehouse) discloses a Time of Flight mass spectrometer with a pulsing region as shown in FIG. 1A. A pseudo potential well is created in the time of flight region by the combination of a pseudo potential barrier formed near the surface 12 of a pusher electrode 11 and a static electric field formed in the time of flight pulsing region by a potential difference applied between the pusher electrode 11 and a counter or extraction electrode 13 (grid electrode). The surface 12 of the pusher electrode 11 comprises an array of electrodes such as a square array of wire tips with neighbouring wire tips alternately connected to opposite phases of a high frequency alternating voltage. An inhomogeneous field is generated creating a pseudo-potential barrier which penetrates a short distance above the pusher electrode 11.    GB 2299446 (Franzen) discloses a multipole rod arrangement as shown in FIG. 3 which is used to orthogonally inject ions into a Time of Flight mass spectrometer.    US 2012/0138788 (Taniguchi) discloses an arrangement as shown in FIG. 1 wherein a plurality of ions are ejected from an ion trap into a Time of Flight mass spectrometer 4.    WO 2011/107738 (Bream) discloses a mass spectrometer as shown in FIG. 4 wherein a transient voltage is applied to an electrode 40 in order to accelerate ions having different masses to approximately equal velocities. The ions are then differentiated at an ion detector by their kinetic energies.    US 2010/0252728 (Mackie) discloses a mass spectrogram employing a set of controllable electrodes to produce a time varying axially inhomogeneous electric field as shown in FIG. 2.    WO 83/00258 (Muga) discloses an arrangement as shown in FIG. 1 wherein ions experience a monotonically time-varying acceleration field.    GB-2486820 (Micromass) discloses a fast pushing time of flight mass spectrometer.    WO 1011/138669 (Albeanu) discloses a triple switch topology for delivering ultrafast pulser polarity switching.    U.S. Pat. No. 4,707,602 (Knorr) discloses a Fourier Transform time of flight mass spectrometer.
It is desired to provide an improved Time of Flight mass analyser.